Films of thermoplastic polymers are well known and widely used in the packaging industry because of their superior physical properties. Included among these well known, commercially available thermoplastic polymers, which have been found to be useful for film applications, are polyolefins, particularly poly(lower) olefins. Exemplary of the conventional film forming polymers of this type are those which have been formed by the addition polymerization of ethylene and propylene individually as homopolymers, together as a copolymer and as copolymers with other, higher olefins, particularly higher alpha olefins.
Polypropylene films, particularly oriented polypropylene films, have found wide acceptance in the packaging arts. This is because of their high tensile modulus and stiffness, their excellent optical clarity, and their good moisture barrier characteristics.
However, one of the advantages of using thermoplastic polymer films in the packaging industry is that they are heat sealable, and oriented polypropylene is not readily heat sealed. Therefore, without modification, this otherwise excellent film would not have wide acceptance in this field. Therefore, the practice has developed of applying a layer of more readily heat sealable material to the surface, or surfaces, of the oriented polypropylene film which are intended to be heat sealed. This layer, which may be applied to a whole surface or any part thereof, has sometimes been called a thermal stripe seal.
Thus, oriented polypropylene film is an excellent packaging material, and, with the stripe seal applied thereto, is nicely heat sealable, and therefore has achieved wide acceptance in the packaging field. This film still has a disadvantages in that, with or without the stripe seal disposed thereon, it has a very high inherent coefficient of surface friction, and a high film to film blocking characteristic, particularly during storage. The high surface friction characteristic causes the film to run with some difficulty, particularly at high speeds, because the film tends to drag against itself and other surfaces.
In the past, these adverse surface characteristics of polypropylene have been sought to be modified by various techniques. In U.S. Pat. No. 4,956,241, there is disclosed the addition, by slip coating a wax emulsion onto the surface of the polypropylene film. In particular, according to this patent, the wax emulsion should be applied to an oriented polypropylene film, and most especially, it should be applied to the film between the machine direction and the transverse direction orientation steps.
The waxes which have been used to improve the surface slip characteristics of the polypropelene film include the usual materials, such as carnauba wax, paraffin wax, polyethylene wax, micro-crystalline wax, and the like. Blends of these waxes are contemplated.
It is also suggested in this patent to possibly use, in combination with the wax, a polymeric binding agent for the wax. This binder is described as a polymer having a glass transition point between about 30.degree. and 100.degree. C. It is said to be used in amounts between about 0 and 50 % of the total weight of the wax. One example of such a polymer which is set forth in an example of this reference is an acrylic resin composed of a copolymer of methyl methacrylate, methacrylic acid and methyl acrylate.
Additional components of the wax formulation include conventional inorganic slip agents, such as silica or talc. Conventional emulsifiers, or suspending agents, and a conventional carrier, such as water, are used to assemble the wax composition into a suitable emulsion or suspension so that it can be applied to the surface of the polymer film.
It has been noted in the past that polypropylene films should be biaxially oriented in order to get the most out of their physical properties. Such orientation may be accomplished by conventional operations, such as bubble orientation, or tenter frame orientation. The particular type of orientation which is used is a matter of choice to the operator. However, where slip agents have been applied in the past, there seems to have been a preference for the use of a sequential machine direction/transverse direction orientation procedure, with the slip agent applied between the two orientation steps.
As noted above, polymeric films which are suited to use in the packaging industry, are valuable for many reason. Not the least of the values of using thermoplastic films for packaging is the fact that these films are heat sealable. However, it has been found that the modification of the surface characteristics of polypropylene films to improve their slip characteristics, by the application of wax thereto, has also degraded the heat seal which is accomplished through the thermal stripe seal described above. That is, the successful efforts which have been made in order to improve the film surface slip characteristics, and to reduce the blocking tendency thereof, particularly the tendency to block on storage of oriented polypropylene films, have degraded the heat sealability of these films through the above referred to thermal stripe. Of course this is undesirable.